Novel antimony phosphates with enlarged birefringence induced by lone pair cations.

Phosphates, whose obvious disadvantage is the relatively small birefringence, can be overcome by the introduction of post-transition metal cations containing stereochemically active lone-pair electrons. In this paper, two new compounds were successfully explored in the A-Sb-P-O system, i.e. Cs 2 Sb 3 O(PO 4 ) 3 (CsSbPO) and (NH 4 ) 2 Sb 4 O 2 (H 2 O)(PO 4 ) 2 [PO 3 (OH)] 2 (NH 4 SbPOH). Transmission spectra show that CsSbPO has a surprising transmission range with a UV cutoff edge of 213 nm. First-principles calculations show that both compounds have a wide band gap (5.02 eV for CsSbPO and 5.30 eV for NH 4 SbPOH) and enlarged birefringence (Δ n = 0.034@1064 nm for CsSbPO and Δ n = 0.045@1064 nm for NH 4 SbPOH). The results of real-space atom-cutting investigations show that the distorted [SbO x ] polyhedra originating from the asymmetric lone pair electrons give the main contribution to the total birefringence and overcome the disadvantage of small birefringence of phosphates but maintain wide transition windows.